This invention relates to optical etalons, and particularly to etalons that are to be used in fiber optic communications and fiber optic wavelength division multiplexers and demultiplexers.
Various types of interleaver and deinterleaver devices are used in WDM and DWDM systems for multiplexing and for demultiplexing. These devices include optical gratings, thin film optical interference filters, air-gap etalon filters, and Gires-Tournois filters.
FIG. 1 shows an air-gap etalon. The air-gap etalon has two plates 10 and 11 that are spaced apart by spacers 14 and 15 so that the optically flat surfaces 12 and 13 of the two plates are parallel and face one another across an air-gap. The plates are transparent to the wavelengths of interest and the optically flat surfaces 12 and 13 are partially reflective so that light may be reflected many times between the partially reflective surfaces before exiting through one or other of the plates. If one of the reflective surfaces is 100% reflective, then the etalon is called a Gires-Tournois interferometer. The etalon has the characteristics of a comb filter in that it passes light in a plurality of evenly spaced passbands. The passbands are evenly spaced or separated by the free spectral range (FSR) of the etalon. The FSR is inversely proportional to the optical distance between the mirrors. The physical distance between the mirror surfaces is d, as indicated by the double headed arrow 16, and the optical distance is nd, where n is the refractive index of air.
A Gires-Tournois interferometer (GTI) that is to be used in WDM or DWDM must have passbands that match the standard wavelengths established by the International Telecommunications Union (ITU). At room temperature, it is required that the resonant wavelengths of the GTI match the ITU standard wavelengths to within 0.01 nm or 1 part in a million. Therefore the optical length of the GTI must also be within 1 part in a million of the ideal value. It is not feasible to make spacers to such a tolerance. Therefore, tuning is required to adjust the optical length of a GTI to bring the optical length within the tolerances stated above.
A number of methods have been used for altering the optical distance between the mirrors of an etalon to adjust or tune the pass bands. Such methods include the use of pneumatic cell, piezoelectric devices, and rotating the etalon.
Pneumatic cell and piezoelectric tuning devices are unsuitable for DWDM because they are active devices and also have a problem of temperature instability
FIG. 2 illustrates conventional tuning by etalon rotation. The etalon as a whole is rotated through a small angle xcex8 relative to the incident beam, thus changing the effective optical length of the etalon. In WDM and DWDM, the etalon must be perpendicular to the incident light beam, and therefore etalon rotation is not a suitable method of tuning.
In designing an etalon or GTI for use in DWDM, special attention must also be paid to the requirements for temperature stability. The drift of the center (resonant) wavelengths must be less than 0.01 nm from 0 to 65xc2x0 C. In an air spaced etalon, the thermally induced drift of center wavelength is due to thermal expansion of the spacer material, and is also due thermal effects in the air.
The density of the air is       ρ    =          p      RT        ,
where R is the gas constant and p is the pressure. If the air is allowed to expand at a fixed pressure, then             ⅆ      n              ⅆ      T        ∝      -          p              RT        2            
negative and its magnitude is ≈10xe2x88x926. This causes about 0.1 nm wavelength drift when the temperature changes from 0 to 70xc2x0 C. This problem is avoided by putting the etalon in a sealed enclosure where the air density is essentially constant and the index of refraction remains essentially constant as temperature changes.
Thermal expansion of the spacer material remains as the main cause of thermal drift of center wavelengths. Various techniques for compensating the thermal expansion of the spacers are known. For example, placing one of the mirrors on a thermally expanding riser is known.
FIG. 3 shows an etalon in which one of the mirrors is located on a riser. The optically flat mirror surface 13 is here located on a riser 17. As the temperature increases the spacers 14 and 15 expand to make the optical distance between the mirrors longer, while the riser expands to make the optical distance between the mirrors shorter. With proper choice of materials for riser and spacers the expansion of the spacers can be compensated. The use of risers increases the complexity of manufacturing an etalon and does not provide for tuning.
It is an object of the present invention to provide a method for accurately tuning an etalon to ITU standard wavelengths for fiber optical communications.
It is an object of this invention to provide an etalon tuned to ITU standard wavelengths wherein the tuning is accomplished by means of a tuning plate in the etalon.
It is an object of the present invention to provide a tuned etalon with a tuning plate, wherein the temperature stability of the etalon spectrum is enhanced by compensating spacer thermal expansion by the use of a selected optical glass in the tuning plate.
It is a further object of the present invention to provide a tuned etalon in which the tuning plate provides dispersion compensation.
The objects and advantages of the present invention are obtained by an etalon in which a tuning plate provides both a course and a fine adjustment of the optical length of the etalon cavity to bring the etalon output spectrum into very close alignment with ITU standard channels.
The method of the invention includes selecting a spacer length that is slightly less than ideal, placing a first tuning plate of known thickness and refractive index in the etalon, measuring the FSR of the etalon, determining the required additional tuning plate thickness to bring the etalon exactly into tune, selecting and installing a second tuning plate of the same material that is slightly less than the ideal thickness, and then fine tuning the etalon by rotating the second tuning plate through a small angle until the etalon spectrum is the same as the ITU standard to within a required tolerance.
The etalon of the present invention is an etalon in which a tuning plate is located in the air-gap, and in which the effective thickness of the tuning plate has been adjusted by rotating the tuning plate to an angle at which the output spectrum of the etalon coincides with ITU standards to a required tolerance.
The etalon of the present invention includes a tuning plate that preferably has a small effective coefficient of thermal expansion so that the thermal expansion of etalon spacers is compensated or partially compensated over a range of temperatures.